ComputerAided_Design_Engineering_amp_Manufactur.pdf

adjusted and changed automatically by robotic devices without human intervention. The requirements
for locating and constraining workpieces using such fixture modules are presented. Furthermore, the
fundamental issues for planning and analysis of fixture setup are described. A technique for computeraided
planning and analysis of reconfigurable fixture is also presented in this chapter.
3.1 Introduction
Workpiece positioning and constraining, commonly referred to as workholding or fixturing, is an important
issue in manufacturing processes. 1 In general, for any manufacturing operation to be successful, the
workpiece must be located and held to remain in a desired position and orientation when subjected to
external forces during manufacturing operations. Jigs and fixtures are the devices employed to position
and constrain the workpiece for presentation to the manufacturing device. The traditional approach to
fixturing involves designing and manufacturing special-purpose fixtures for specific workpieces and
manufacturing operations. 2 In addition to this expensive process, a significant amount of intervention
by skilled operators is needed in setting up and changing the fixtures when the manufacturing operation
is completed or modified. Therefore the traditional approach is generally time consuming and thus costly.
This is especially true for current and future manufacturing requirements characterized by rapid change
of product design and reduced production cycle time.
To reduce the dependence on dedicated fixtures, recent research efforts have been directed toward
developing alternative approaches. Several flexible fixturing strategies have been proposed and developed. 3
These strategies include sensory-based techniques, modular fixturing, reconfigurable fixturing, conformable
clamps, and many others. It is believed that reconfigurable fixturing is one of the most appropriate
strategies in a computer-integrated manufacturing (CIM) environment. However, for any fixturing
technique to be effective, it must be supported by a computer-based planning and analysis system. This
chapter will briefly describe the economic justifications for flexible fixturing within a computer-integrated
manufacturing (CIM) environment, and, more importantly, within flexible manufacturing systems.
Flexible fixturing strategies will be briefly described and fixture design requirements will be presented.
However, the main focus will be on requirements, guidelines, structure, and formulation for the CADbased
planning and analysis of fixture configuration. These will be described in detail.
3.2 Economic Justification
A great deal of attention has been directed towards the development of advanced manufacturing
systems. Manufacturing systems can be classified into two distinct categories: high volume and lowto
medium-volume production systems. It is quite apparent that investments in manufacturing
technology are more easily justified in high-volume production environments than in low- to mediumvolume
environments. High-volume production systems typically produce a large number of similar
parts with similar processes. Therefore, factors justifying such investments in automated manufacturing
technology include:
• Large production batches of same parts with similar processes
• Very few design changes over a long period of time
• Reduction of labor costs for support functions (such as materials handling and part positioning
and constraining) by the deployment of dedicated and hard-tool fixturing
• Simplified scheduling in conjunction with less need for program development due to the repetitive
nature of tasks and large batches.
On the other hand, low- to medium-volume production systems produce a larger number of diverse
parts in much smaller batch sizes. 4 Therefore, in low- to medium-volume production systems, factors
that prevent investments in automated manufacturing technology are basically the reverse of those in
the high-volume production systems:
© 2001 by CRC Press LLC